1. Field of the Invention
The invention relates to a vehicle power transmission system that suppresses deterioration of the drivability of the vehicle when the vehicle power transmission system is switched from a non-drive mode to a drive mode.
2. Description of the Related Art
A control device of vehicle power transmission system is known, which has a first electric motor, a differential section that is provided with a rotational element connected to the first electric motor to control a differential motion between an input rotation to the differential section and an output rotational speed of the differential section by controlling the operation state of the first electric motor, a power interruption element that forms a part of a power transmission path, a second electric motor that is connected to the power transmission path between the power interruption element and the output rotational element of the differential section. For example, according to a control device of a hybrid vehicle in Japanese Patent No. 3346375, a control is performed such that power transmission from a power source is interrupted during the period when the power transmission system is switched from a non-drive mode to a drive mode. Thus, switching toward the drive mode may be smoothly performed while shift shocks is effectively suppressed, regardless of variations of the output state in the power source due to variations of the accelerator depression amount or switching of the operation mode.
With regard to power transmission system having a differential section as described above, the inventors have found that the output rotational speed of the differential section needs to be maintained constant, preferably, fixed at zero when the engine is being started up or when a load operation is performed for power charging via an electric motor while the engine is being operated at a non-drive range (e.g., P range and N range). However, according to related arts as described above, when the shift range is changed from a non-drive range (e.g., P range and N range) to a drive range (e.g., D range and R range), it may result in an increase in the output rotational speed of the differential section, causing engagement shocks, abnormal engine noise, and so on because the constant output rotational speed of the differential section is not maintained any longer so as to establish the drive state of the power transmission system before the engagement of the coupling element is completed.